The present invention relates to a method for manufacturing an internal combustion engine piston of the two-piece, or articulated, type. In particular, the present invention provides for a method for making the upper, or head, portion of such piston.
The two-piece, or articulated, piston is composed of two parts: the upper part, or head, intended to transmit the pressures generated by the engine combustion, and the lower part, or skirt, whose function is to guide the upper part in the cylinder. The skirt is mounted on a pair of pin bosses depending from the top portion by means of a wrist pin bearing on pin holes provided at the lower ends of the pin bosses.
In high output diesel engines, where the pressures and temperatures within the cylinder are severe, reaching as much as 2,500 psi and 1,000.degree. C. respectively, the upper part of articulated pistons is generally obtained by machining a billet of chromium steel or a low alloy steel, in most cases forged to a shape requiring very little machining to the desired final dimensions. According to the conventional methods, the billet of the selected material is forged in the shape of a cylindrical member integral with two legs depending therefrom known as pin bosses at the ends of which, opposite the cylindrical member, are provided aligned holes. Owing to the said thermomechanical loads, it is customary to provide the underside of the head of such pistons with a peripheral groove which, in association with a peripheral tray or channel on the top of the skirt, will define a cooling chamber designed to remove parts of the heat from the head during engine operation.
However, it has been a difficult task to obtain the final shape of the head for, in addition to the provision of a combustion chamber on the top portion to improve combustion and a plurality of peripheral grooves for the piston rings, there is also the need for carrying out the machining of the top underside to provide the peripheral groove for the cooling chamber. This is a costly process and its application is limited to axisymmetrical chambers, i.e., chambers in which the peripheral groove is concentric with the longitudinal axis of the head. The attempts made heretofore to provide the peripheral groove during the forging operations have resulted in parts having an unacceptable dimensional instability on that portion of the head provided with grooves for the piston rings. In addition, a high rate of tooling damage has been observed.